The present invention relates to cavity ringdown spectroscopy, and more particularly, but not exclusively relates to servo-switched frequency control of a light source for an optical resonator.
Cavity Ringdown Spectroscopy (CRDS) is a technique that, among other things, is directed to making absorption measurements with sensitivities that can be better than many other systems. In a typical CRDS system, the optical resonator includes two or more mirrors in an optical cavity aligned so that incident light circulates between them. The sample of absorbing material is placed in the cavity for interrogation. When input light to the resonator is discontinued, the radiant energy stored in the resonator decreases over time or “rings-down.” For an empty cavity, the stored energy follows an exponential decay characterized by a ringdown rate that depends on the reflectivity of the mirrors, the separation between the mirrors, and the speed of light in the resonator. If an absorbing sample is placed in the resonator, the ringdown changes under appropriate conditions such that the resonator energy decays in a measurably different fashion than that for the empty resonator. A corresponding absorption spectrum for the sample is obtained by plotting the reciprocal of the ringdown rate versus the wavelength of the incident light. CRDS has been applied to numerous systems in the visible, ultraviolet, and infrared wavelength regimes. U.S. Pat. No. 5,528,040 to Lehmann is referred to as an additional source of background information concerning such techniques and is hereby incorporated by reference.
Current CRDS systems typically use a pulsed laser or a Continuous Wave (CW) laser. With a pulsed laser, the pulse length is generally much shorter than the ringdown time, so that less light enters the resonator compared to CW laser-based approaches. As a result, pulsed laser CRDS generally is less sensitive than CW laser CRDS. On the other hand, with CW laser CRDS, the need to switch the resonance off and on can be more complicated. Such switching is often performed with an Acousto-Optic Modulator (AOM), which can change the amplitude, frequency, and/or direction of the laser beam provided to the optical resonator, any one of which can be altered to initiate ringdown. Because an AOM provides a traveling wave with an acoustic grating that takes some time to form and dissipate, AOM switching typically has intrinsic speed limitations. These limitations can result in inaccuracies of the exponential fit of ringdown data, especially in the mid-wavelength to long-wavelength infrared ranges for which the AOM device is typically bigger and slower. For such wavelengths, the optical cavities of the corresponding resonators often have a lower resonance quality or “Q” factor, and correspondingly dissipate the decaying optical field faster during ringdown. In addition to drawbacks regarding the switching process, experiments of a more precise nature typically desire to “lock” the laser frequency to a selected resonant frequency, which is lost at the beginning of each ringdown interval and must be reacquired at the beginning of each excitation interval. These frequency lock techniques typically increase the complexity of the switching schemes used with CW laser CRDS.
Accordingly, there is a need for further contributions in this area of technology.